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Examples of General Rock Strength by Area   

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Overview 

Rock strength varies widely by type as well as by individual composition characteristics. The rock at any given area can be composed of many different layers, each with a slightly different mineral composition and density. Thus, the exact compressive strength of a rock is difficult to predict based solely on its type. A sample of each unique layer would have to be taken and tested in a lab to know the exact strength. However, the following chart provides a range of expected compressive strengths measured in pounds per square inch per rock type of those rock types commonly found in the U.S.

Rock Type Compressive Strength Range (psi)
  • Granite 14,500-36,250
  • Diorite 21,750-43,500
  • Diabase 14,500-50,760
  • Basalt 14,500-43,500
  • Gneiss 7,250-29,000
  • Slate 14,500-29,000
  • Quartzite 21,750-43,500
  • Sandstone 2,900-24,660
  • Shale 725-14,500
  • Limestone 4,350-36,250
  • High Strength Concrete 10,150
  • Diamond 6,425,000
from stanford.edu/~tyzhu/Documents/...

The chart above demonstrates the wide range of rock strengths both between and within rock types. Due to this variability, general rock type is not the sole determining factor in choosing an ideal anchor bolt configuration.

The following chart outlines the strength of general categories of rock based on their usual compression strength:

Hard Rock ≥14,500 psi
Medium Rock 7,250-14,449 psi
Soft Rock 1,800-7,249 psi

A core sample and hydraulic laboratory testing equipment are needed to accurately determine the compressive strength of a given rock. However, the following characteristics can be used to help make a reasonable estimation of rock hardness in the field:

Hard Rock: cannot be scratched with a knife; requires multiple heavy hammer blows to deform
Medium Rock: scratches with a knife only under heavy pressure; deforms with one heavy hammer blow
Soft Rock: scratches under light to moderate pressure; deforms with one moderate hammer blow

(Please do not use the above testing methods on the cliff itself. Instead, find a loose rock that appears to represent the layer in question from the cliff base.)

Examples of General Rock Strength by Area 

Hard Rock
New River Gorge, WV – Nuttal sandstone
Eldorado Canyon, CO – Foundation arkosic sandstone
Joshua Tree, CA – White Tank monzogranite
Gunks, NY – Shawangunk conglomerate
Linville Gorge, NC – Chilhowee quartzite
Tennessee Wall, TN – Gizzard sandstone
Rumney, NH – Littleton schist
Seneca, WV – Tuscarora quartzite
Devil’s Lake, WI – Baraboo quartzite
Longs Peak, CO – Silver Plume granite
Grand Teton, WY – Mount Owen quartz monzonite
Washington Pass, WA – Golden Horn granite

Medium Rock
Cumberland Plateau sandstone
Whiteside, NC – Tanolite/quartz diorite
Rifle, CO – Leadville limestone
Wild Iris, WY – Bighorn dolomite
Paradise Forks, AZ – Basalt
Smith Rocks, OR – Welded tuff
Indian Creek, UT – Wingate sandstone
Owens River Gorge, CA – Bishop tuff

Soft Rock
Zion, UT – Navajo sandstone
Fisher Towers, UT – Cutler sandstone
Red River Gorge, KY – Corbin sandstone
Arches NP, UT – Entrada sandstone
Red Rocks, NV – Aztec sandstone

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Comments on Examples of General Rock Strength by Area Add Comment
By 20 kN
Administrator
From: Hawaii
Feb 11, 2016
The Rock Type Compressive Strength Range is off. For example, sandstone can be far softer than the quoted minimal value of 2,900 PSI. In reality, soft sandstone can be as low as 500 PSI or even lower for low-quality sandstone.